1. Statement of the Technical Field
The present invention relates to the field of dielectric lens antennas, and more particularly to dielectric lenses using fluidic dielectrics.
2. Description of the Related Art
Dielectric lens antennas are used as a means for controlling the directivity of radio waves. FIG. 1 is a cross section of a conventional dielectric lens antenna. This conventional dielectric lens antenna 1 comprises a dielectric lens 2, a primary radiator 3, and a dielectric member 4 having a lower dielectric constant than the dielectric lens 2, provided between the dielectric lens 2 and the primary radiator 3. The dielectric lens 2 is typically but not necessarily disk shaped with a lenticular section. The primary radiator 3 is disposed at the back focal point of the dielectric lens 2. The dielectric member 4 is typically formed in a substantially circular cone shape in which the primary radiator 3 is positioned at the apex, and the dielectric lens 2 is provided at the base, and its dielectric constant is uniform. Further, the dielectric lens 2 and the primary radiator 3 are connected through and secured to the dielectric member 4. In the dielectric lens antenna 1, the thickness of the dielectric lens 2 can be reduced, and moreover, it is unnecessary to provide a holder for holding the dielectric lens 2 at a predetermined position with respect to the primary radiator 3. (There also exist embodiments in which the intervening dielectric media 4 consists of free space and the relative positions of the primary radiator 3 and the dielectric lens 2 are maintained by external solid structures not shown, or in which the primary radiator 3 and the dielectric lens 2 abut. The proposed invention is applicable to these embodiments as well.) For reduction of the thickness of such a dielectric lens antenna, U.S. Pat. No. 6,356,246 discusses increasing the dielectric constant of a dielectric lens in order to make the dielectric lens thinner, shortening the back focal distance of the dielectric lens so that the distance between the dielectric lens and the primary radiator is reduced, or increasing the dielectric constant of a dielectric member so that the distance between the primary radiator and the dielectric lens is reduced, and so forth. However, when the dielectric constant of a dielectric lens is increased, the efficiency of the dielectric lens itself is reduced. Further, to reduce the back focal distance of the dielectric lens, it is necessary to increase the thickness of the dielectric lens, and as a whole, the thickness of the dielectric lens antenna can not be reduced. Further, this causes the problem that the efficiency deteriorates. Further, since materials with which dielectric lenses are formed have high heat shrinkage, dielectric lenses which are thick can not be injection-molded with high dimensional precision. In the methods for increasing the dielectric constant of the dielectric member, phase-shifting increases, due to the routes of radio waves between the primary radiator and the dielectric lens. Accordingly, there is the problem that the dielectric lens antenna can not operate normally. Furthermore, existing systems such as the dielectric lens system discussed in U.S. Pat. No. 6,356,246 are static. In other words, they are designed and optimized for a single frequency or application and fail to provide a broader range of applications in terms of operational frequency and size variability. Thus, a need exists for a dielectric lens that overcomes the problems discussed in U.S. Pat. No. 6,356,246 and further provides greater range of operation.
Two important characteristics of dielectric materials are permittivity (sometimes called the relative permittivity or εr) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (με)}. The propagation velocity directly affects the electrical length of a transmission line and therefore the amount of delay introduced to signals that traverse the line.
Further, all dielectric structures have a property known as characteristic impedance, which expresses the relative amplitude of electric and magnetic fields in a propagating electromagnetic wave. Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to √{square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the transmission line structures as well as the physical geometry and spacing of the line structures. For unguided plane waves propagating in a dielectric medium, the characteristic impedance is η0√{square root over (μr/εr)}, where η0 is the impedance of free space.
The purpose of a dielectric lens 2 is to control the delay of waves propagating through the lens at various points in order to control the direction of energy radiated from the dielectric lens antenna 1, or in the case of a receiving antenna, to control the directive response of the antenna. For a lens made of a given dielectric material, the profile of the lens is shaped to achieve the desired delays. The higher the dielectric constant of the lens material, the thinner it can be to achieve the desired delays. However, if the impedance of the lens material is radically different from the impedance of free space or of the material comprising the dielectric member 4, wave reflection losses at the material interfaces may be unacceptably high.